Topic: A close look at the jet needle, and why aftermarket doesn't cut it

[fizzlebottom]

As my profile shows, I have a 1982 CB650SC Nighthawk. This bike came with Keihin VB44C (CV) carbs. Everything about this bike and these carbs was originally setup to be a very user-friendly (non-adjustable) experience. This bike also has mostly single-year single-model parts, and today's parts availability shows.

On to the carbs. When we all get rebuild kits and inevitably throw away the garbage brass, we've learned that it is because aftermarket companies don't really adhere to the same tolerances that Keihin did, or they just design to cover as many bases as possible. This applies to the jet needle as well as the other jets. How do I know?

Because I've attached close-up pics of both an original Keihin 64A jet needle and an aftermarket one meant to fit my carbs. At 40 years old, the Keihin needles might be a little worn, but that doesn't account for the WILD difference in the tips. The aftermarket needles have a 2-angle profile and a very blunt tip, while the original needle has a single long angled profile and a less blunt tip (possibly worn?).

Now, I honestly do not know the specifics on how these factors play into running conditions across the RPM range. But, I am imaging that the differences can be noticeable especially when paired with aftermarket mains or slow jets.

We so often see mentions of how aftermarket main and slow jets have holes in different spots than OEM, or are different lengths, or even bad threads. I think the needle is sort of overlooked or not discussed enough. And even though I'm focusing on CV carbs, similar (or even worse?) problems can show up with PD carbs when poorly matched needles & needle jets are used.

Discuss.

[denward17]

I don't know enough to have a discussion on these needles, but I can see where it would definitely affect low speed throttle conditions, and initial turn up.

A long time ago I had 28mm Mikuni carbs (Kawasaki 900) bored to 29mm, and had a fun time with jets/needles but was able to find a good match and had the bike running like stock off idle.   Back then there were plenty of options to get quality parts though, probably not so much now.

[HondaMan]

The aftermarket parts since about 2002 have been made up from whole cloth, from what I can see, for these bikes' carbs. The needle jets and jet needles have no resemblance to the OEM parts, so you are left to figure out your jetting when using them, all by yourself. The emulsifier holes in those parts are easily copied from the OEM parts, but in some (CB500/550 for example) the hole size in the needle jet matches the type of needle used, whether Keihin or (aftermarket]. Then you're kind of stuck.

I can offer this much for the CB750K0-K6 & F0 roundtop carbs: if using Keyster parts, the mainjet must be 30%-40% larger than your OEM number and the idle jet 10% bigger.

If using the 750 PD carbs, you must likewise use similarly larger mainjets and will likely have to raise the needle in the slide by 2mm to get an idle-to-mainjet transition that won't throw you over the handlebars.

[seanbarney41]

I don't know enough to have a discussion on these needles, but I can see where it would definitely affect low speed throttle conditions, and initial turn up.

A long time ago I had 28mm Mikuni carbs (Kawasaki 900) bored to 29mm, and had a fun time with jets/needles but was able to find a good match and had the bike running like stock off idle.   Back then there were plenty of options to get quality parts though, probably not so much now.

I have had similar experiences getting vm28's to work excellent on a hopped up kz650 (which stock used vm24's!) This was just a couple years ago.  The Mikuni vm series carbs are used on such a wide variety of engines that there is nearly infinite tuning options for them and most, if not all is still available.  Keihin mostly just cast up a completely different carb for each application, with just one needle profile designed for each.  Changing your exhaust, intake, or cam shaft?  Good luck.  Honda had big stickers on most of their bikes explicitly telling you not to.

[PeWe]

Needle must be right. Different taper and it will not be possible to get it right changing other jets. Allways wrong somewhere else.
Probably both rich and lean depending onho much you throttle.

I have noticed that Mikuni VM series has lots of needles.

It took a while to get correct needles for my TMR carbs. Always out of stock, Webike did not bother to refill eirher.

Below old wrong vs new correct.
The needle with clip is way too rich. (Thinner).

Not possible to get it right by changing.
Pilot air jet, pilot or needle jet.
(The needles in CB750 cheap carb kits have even bigger differences.)

[robvangulik]

Because I've attached close-up pics of both an original Keihin 64A jet needle and an aftermarket one meant to fit my carbs. At 40 years old, the Keihin needles might be a little worn, but that doesn't account for the WILD difference in the tips. The aftermarket needles have a 2-angle profile and a very blunt tip, while the original needle has a single long angled profile and a less blunt tip (possibly worn?).

I think the tip is a less important part of the needle, it is the tapering over the length of the needle that defines the metering over the rpm range, a small difference there can and will cause all sorts of problems getting the carburation right!

[willbird]

The aftermarket parts since about 2002 have been made up from whole cloth, from what I can see, for these bikes' carbs. The needle jets and jet needles have no resemblance to the OEM parts, so you are left to figure out your jetting when using them, all by yourself. The emulsifier holes in those parts are easily copied from the OEM parts, but in some (CB500/550 for example) the lloe size in the needle jet matches the type of needle used, whether Keihin or (aftermarket]. Then you're kind of stuck.

I can offer this much for the CB750K0-K6 & F0 roundtop carbs: if using Keyster parts, the mainjet must be 30%-40% larger than your OEM number and the idle jet 10% bigger.

If using the 750 PD carbs, you must likewise use similarly larger mainjets and will likely have to raise the needle in the slide by 2mm to get an idle-to-mainjet transition that won't throw you over the handlebars.

There is a dude on youtube who made a see through carb for a lawnmower single cylinder engine then he did high speed video of it working with the engine running. I suggested in a comment that he build a carb like that based on a popular Mikuni Carb where his video could show all of the different circuits working.

It would be interesting in a setup like that to compare various available jets and needles.

I think think there is a lot of lost art involved in carbs.

My dad worked at Tillotson Carburator as a tool and diemaker in the late 60's early 70's. he spoke of how production carbs had a slot that was exposed by the butterfly opening up, this changed mixture in the low butterfly opening stages.

In prototype carbs they uses a series of holes of differing sizes to provide the same function that the slot in production carbs would.

He spoke of hearing high rpm small engines running in dyno cells, the engines would scream all day until sometimes they finally blew up and it was quiet until they got the next test engine bolted up.

I had a powerlite chainsaw that had a Tillotson carb on it, the butterfly was probably about 3/8" in dia. My neighbor was playing with chainsaws to annoy the neighbor further down who makes all kinds of racket :-)...I had bought a Milwauki M18 Fuel chainsaw so I gave him that old saw to play with.

Bill

[PeWe]

Do not underestimate the needle. It affect from low.
If wrong the user migh try to compensate with pilot+ air screw/fuel screw making it OK in a narrow band but more wrong elsewhere. Same with the other jets. Needle affect to at least 3/4 lift.

The carb description with jets usually point out needle to work at higher lift than it really does. Mikuni TMR flat slides especially that affect from the very beginning.

Jets affect each other. Still same underpressure so if one gap for a certain underpressure will suck fuel/air will be smaller, more will the other jets contribute.

Needle with needle jet must be correct for carb and engine. The other jets can then be figured out by tests and Dynos.

I have learned the hard way.

Stock CB750 K6 carbs easier to jet even for bigger bore, hotter cam, better flowing exhaust, inlet. Still same needle and pilot jet.

[fizzlebottom]

I think the tip is a less important part of the needle, it is the tapering over the length of the needle that defines the metering over the rpm range, a small difference there can and will cause all sorts of problems getting the carburation right!

Good point. I'll try to get a few measurements of the length and taper at various points for both needles today for your viewing pleasure.

[willbird]

Because I've attached close-up pics of both an original Keihin 64A jet needle and an aftermarket one meant to fit my carbs. At 40 years old, the Keihin needles might be a little worn, but that doesn't account for the WILD difference in the tips. The aftermarket needles have a 2-angle profile and a very blunt tip, while the original needle has a single long angled profile and a less blunt tip (possibly worn?).

I think the tip is a less important part of the needle, it is the tapering over the length of the needle that defines the metering over the rpm range, a small difference there can and will cause all sorts of problems getting the carburation right!

Good point. I'll try to get a few measurements of the length and taper at various points for both needles today for your viewing pleasure.

A really handy tool for checking the taper(s) might be a number drill gauge.

https://testequipmentusa.com/starrett-drill-steel-wire-gage-1-60/

No need for that price point of gauge but it was a nice picture. Should try for one that does not have sharp edges though and use carefully. drop needle in until it rests on hole and measure how much sticks out the top to say the center C clip notch. By checking several dia it might reveal that a needle would work but just needs a different clip setting to achieve the same general profile as an OEM needle.

Today we would do the type of work involved in making needles on a machine called a "swiss turn" but it might be interesting to see how Kehin did it in say 1968.

[Tracksnblades1]

Do not underestimate the needle. It affect from low.
If wrong the user migh try to compensate with pilot+ air screw/fuel screw making it OK in a narrow band but more wrong elsewhere. Same with the other jets. Needle affect to at least 3/4 lift.

The carb description with jets usually point out needle to work at higher lift than it really does. Mikuni TMR flat slides especially that affect from the very beginning.

Jets affect each other. Still same underpressure so if one gap for a certain underpressure will suck fuel/air will be smaller, more will the other jets contribute.

Needle with needle jet must be correct for carb and engine. The other jets can then be figured out by tests and Dynos.

I have learned the hard way.

Stock CB750 K6 carbs easier to jet even for bigger bore, hotter cam, better flowing exhaust, inlet. Still same needle and pilot jet.

+1

The correct slide # is important for your engine build…